#include <iostream>
using namespace std;

template <class T>
void sort(T stu[], int y) {
	for (int i = 0; i < y - 1; i++)
		for (int j = i + 1; j < y; j++)
			if (stu[i] > stu[j])
				swap(stu[i], stu[j]);
}

int main() {
	int classes;
	cin >> classes;
	double medians[100];
	for (int i = 0; i < classes; i++) {
		int c, scores[100];
		cin >> c;
		for (int j = 0; j < c; j++)
			cin >> scores[j];
		sort(scores, c);
		if (c % 2 == 0)
			medians[i] = (scores[c / 2 - 1] + scores[c / 2]) / 2.0;
		else
			medians[i] = scores[c / 2];
	}
	sort(medians, classes);
	if (classes % 2 == 0)
		cout << (medians[classes / 2 - 1] + medians[classes / 2]) / 2.0;
	else
		cout << medians[classes / 2];
	return 0;
}